Electric initiator



June 3, 1969 M. APSTEIN ELECTRIC INITIATOR Filed Sept. 1. 1967 INVENT OR I PSTE/N 14 KM I A-Mwy 5 ATTORNEYS BY W if" yd 201 &

United States Patent O 3,447,416 ELECTRIC INITIATOR Maurice Apstein, Bethesda, Md., assignor to the United States of America as represented by the Secretary of the Army Filed Sept. 1, 1967, Ser. No. 665,692 Int. Cl. F42]: 3/10 US. Cl. 86-1 5 Claims ABSTRACT OF THE DISCLOSURE An inexpensive, easily manufactured electric initiator for fuze detonators which is safe and reliable in operation. The wire bridge of conventional initiators is replaced by a thin-film chemically-deposited nickel-phosphorus resistor having a very small area and deposited on an insulating substrate. The conductor leads are replaced by thin films of metal of good conductivity electroplated over the nickel-phosphorus. The thin-film resistor bridge is coated with lead styphnate by dipping the end of the substrate into a slurry of the explosive using slightly modified paper-matchmaking machinery.

The invention described herein may be manufactured, used, and licensed by or for the Government for governmental purposes without the payment to me of any royalty thereon.

BACKGROUND OF THE INVENTION In fuzes for small weapons, one of the most expensive elements is the initiator and the circuitry necessary to initiate firing thereof. The initiator is usually followed by a small explosive charge which serves to detonate the major explosive. Until this invention there has been lacking a means of producing an initiator which was inexpensive, easy to manufacture by mass production methods known to industries other than the explosive industry, and reliable in operation and safe to handle. In minimizing the cost of manufacture of an electric initiator it is important that the components, especially the initiating circuitry, allow rapid automatic processing, and that safety not be compromised. To allow rapid automatic construction of the initiator it is necessary that the construction require as few operations as possible and those production operations that are necessary be capable of being done in accordance with mass production techniques.

It is therefore an object of the present invention to provide an electric initiator that is capable of being constructed by high speed manufacturing techniques.

A further object of the present invention is to provide an electric initiator that can be safely and rapidly manufactured with existing or only slightly modified machinery.

A further object of the invention is to provide an electric initiator that can be used with existing components to allow manufacture of inexpensive reliable and safe detonators.

SUMMARY OF THE INVENTION Briefly, in accordance with the present invention, the wire bridge initiator of conventional detonators is replaced by a conductive film or/a thin-film chemicallydeposited electroless nickel-phosphorus resistor bridge having a very small area and deposited on an insulating substrate. In the thin-film embodiment, the conductor leads are replaced by thin films of electroplated-copper deposited over the nickel, and the thin-film resistor bridge area is coated with lead styphnate by dipping the end of the substrate into a slurry of the explosive. The dipping step can be inexpensively accomplished using existing or 3,447,416 Patented June 3, 1969 only slightly modified paper-match-making machinery, and the overall shape of the explosive tip gives the device the appearance of a match.

BRIEF DESCRIPTION OF THE DRAWINGS The specific nature of the invention, as well as other objects, aspects, uses and advantages thereof, will clearly appear from the following description and from the accompanying drawing, in which:

FIGURE -1 is a longitudinal section of an initiator in accordance with the present invention.

FIGURE 2a, b, and c are perspective views showing three stages in the manufacture of a second embodiment of the invention.

FIGURE 3 is a magnified view of said second embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT In FIGURE 1, a flat insulating board 11 is covered on the top and bottom by a flat copper coating 12. The right end of board 11 has a conductive film 13 applied thereto. Conductive film 13 extends to both copper strips 12 to allow a current to be transferred from one copper strip to another, and could be any of the well known conductive mixes (e.g. Aquadag) available to the skilled worker in the art. Surrounding conductive film 13 is a suitable primer mix 14 such as lead azide or lead styphnate. An explosive mix 15 surrounds primer mix 14 and could be of any of the well known explosive mixes such as RDX or tetryl.

The electric initiator of FIGURE 1 has a great advantage over prior art initiators in ease of manufacture. A copper clad insulating board 11 of a large width is used in the preferred manufacture of my invention. The insulating board 11 is sliced to define individual units, and the ends are then dipped into a conductive-mix slurry and removed therefrom, letting the conductive film 13 dry. Successive dippings in a primer mix 14 and explosive mix 15 complete the coating of the initiator. After the initiator has been coated, the individual units of board 11 can be completely severed to establish a plurality of individual initiators. The method I have just described can be accomplished with only slight modification of conventional match-making techniques.

If it were desirable to change the resistance between the flat copper strips, the strips can be pinched toward each other to decrease the gap therebetween at the end where the conductive film is deposited.

In FIGURE 2a, a nickel-copper plated sheet of polyethylene 70 is utilized to make electrical initiators using electroetching and match-making techniques. While any smooth-surface high-density insulating material with sufficient thickness to be self supporting can be used as a substrate for the subsequent nickel and copper plating, the following procedures are effective with polyethylene to form the plated sheet.

To obtain a water-clean free surface smooth enough for electroless plating, the sheets of the unplated insulating material 70 should be immersed in a chromic acid solution at C. for about ten minutes. They are then rinsed with tap water once and with distilled water three times and placed in distilled water until the next operation.

The cleaned polyethylene sheets are then placed in a sensitizing bath for a few seconds and rinsed well with distilled water. They are next placed in an activating bath for a few seconds in preparation for the plating process. After rinsing, the sheets are immersed in a pre- C. was found to be preferable for successful operation of the process. The thickness necessary to be plated on the polyethylene sheet is in the order of .001 inch. After the nickel plating has been accomplished, sheet 70 is washed in distilled water and dried at about 80 C.

A thin film of copper is plated on top of the nickel. This can preferably be done by placing the nickel-coated sheet in a copper-plating bath using the nickel-coated sheet as a cathode and a bar of copper as the anode. After the copper plating is completed, the sheet with its plated surfaces should be rinsed in tap water. This results in the nickel-copper plated sheet of polyethylene 70 shown in FIGURES 2a and b.

The plated polyethylene sheet 70 is covered with resist by dipping the sheet into a solution of two parts of KMER (Kodak Metal Etch Resist) thinner and one part of KMER resist. The resist-coated, nickel-copper plated polyethylene sheet is then dried at approximately 80 C. for 15 minutes. A photographic transparency of a first composite resistor-conductor pattern, shown in FIGURE 2a as areas 53, is then placed on one side of the plated polyethylene and ultraviolet light is allowed to pass through the transparency for approximately 3 minutes exposing the resist except under the opaque portions 53 of the pattern. The resist is developed in a KMER developer, washed with cold water, and dried at 80 C. for approximately 15 minutes. This step can be repeated to reinforce the process. This development step removes the resist from areas 53. The strips are then immersed in an acid ammonium persulphate etching bath at room temperature and the unprotected nickel and copper it etched away from areas 53. The remaining resist is swabbed away with a developer solvent.

In FIGURE 2b a second coat of resist is applied in the manner described above, and the areas 54 are covered with the opaque portion of another photographic transparency pattern. After exposure to ultraviolet light and processing the resist in the same manner as before, the copper from areas 54 is removed by etching in a chromic acid bath. This stage of the etching process does not etch the nickel plate under the copper and a thin film of nickel will remain.

To complete manufacture of the initiator, the sheet 70 shown in FIGURE 2b is sliced along lines 76 to define individual units 51 which are still attached at one end of the sheet 70. These units are separated by standard match making procedures and dipped about the end where the etching took place in a slurry containing a high-sensitivity low-order explosive 65 such as lead styphnate as previously described. After the dipping has been accomplished the strips 51 can be separated from the sheet as is a match from a standard book of paper matches, and used as individual explosive initiators. Such a completed individual initiator is shown in FIGURE 2c.

The operation of the initiator can be easily understood from the magnified view of the individual unit 51 shown in FIGURE 3. Insulator 56 is plated with a nickel layer and copper layer 61. The nickel and copper has been etched from areas 53' by the method described above.

Similarly the copper above area 54 has been etched away leaving the nickel bridge 54 connecting the copper sections 55 and 57. When it is desired to ignite the initiator, voltage is applied to the two copper sections 55 and 57 by cofinecting a suitable voltage potential between the top and bottom of the unit 51. Current flows through the nickel bridge 54 and increases its temperature to such a great degree that the temperature will reach the initiation temperature of the explosive 65. It is important to realize that the geometry of the thin-film bridge 54 must be such as to eifectively perform its functions. It must be of such geometry that the energy available in a firing capacitor will be sufficient to raise the temperature of the bridge plus a critical volume of the explosive coating to the detonation temperature of the explosive. The bridge and conductor materials must not chemically react with the explosive even after extensive storage at elevated temperatures.

I claim as my invention:

1. A method of manufacturing an explosive initiator comprising the steps of:

(a) plating an electrically insulating substrate material with an adherent film of high resistance conductive material,

(b) applying an over-film of low resistance conductive material to said high resistance conductive material,

' (c) chemically etching the high and low resistance conductive materials so as to form a thin-film high resistance bridge connecting two sections of the low resistance conductive material, the said two sections being terminals for the application of a voltage potential to the initiator, and

(d) dipping the etched part of the plated substrate into an explosive mix so that the explosive mix is deposited over the high resistance bridge.

2. A method as described in claim 1 wherein the plated insulating substrate is a large sheet with a plurality of individual initiator circuits etched thereon.

3. A method as described in claim 2 wherein the large sheet is cut into individual units and the explosive mix is deposited onto said units.

4. A method as described in claim 3 wherein the high resistance conductive material is nickel.

'5. A method as described in claim 4 wherein the low resistance conductive material is copper.

References Cited UNITED STATES PATENTS 2,747,257 5/1956 Ashcroft et al 29-623 2,891,477 6/1959 Swanson 102-28 2,995,086 8/1961 Scott 102-28 3,194,160 7/ 1965 Spillane et al 102--28 3,353,262 11/1967 Ingold et a1 86-1 X VERLIN R. PENDERGRASS, Primary Examiner.

US. Cl. X.R. 

